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Abstract

The effects of long chain fatty acids (LCFAs) on H2 -producing mixed anaerobic cultures were evaluated using kinetic, biochemical, molecular biology and statistical methods. The objectives of this study were to assess changes in microbial diversity and metabolic pathways by exposing the cultures to LCFAs. Batch experiments were conducted at 37°C and initial pH 5.0 containing a mixed culture fed glucose and LCFAs (linoleic, palmatic, myristic, lauric, or a 50:50 mixture of myristic and palmatic acids). The H 2 yields for cultures from different sources were statistically the same when LA was added (3.3 mole H2 ·mole-1 glucose; conversion efficiency = 82.5%). LA inhibited H2 -consumption, propionate production, and directed the electron flux towards acetate:butyrate fermentation. Clostridia, Bacteroides, Flavobacteria, Syntrophus, Geobacillus, Actinobacillus, Citrobacter, Cytophaga, Enterobacter, Erwinia, E. coli, and Klebsiella dominated the LA-treated culture while Bacteroides, Bacillus, Clostridia, Acinetobacter, Falvobacteria, Eubacteria, and Rubrobacter dominated the control culture. When LA was added at an initial pH of 5.0, the H2 specific consumption rate decreased by approximately 99%; however, only a 24% reduction was detected in controls when the initial pH was set at pH 5.0. In the presence of LA, acetate, propionate, and butyrate degradation were inhibited. LA and its degradation by-products (PA, MA, and LUA) sustained higher H2 yields for 30 days in suspended cultures but not in granulated cultures. LA affected the diversity of the microbial communities differently in suspended and granular cultures from the same origin. After LA-treatment, the suspended culture showed a greater diversity of acidogens than the granular culture. Adding PA increased the H2 yield; however, LCFAs shorter than 16 carbons did not show any H2 production. Thermoanaerovibrio, Geobacillus and Eubacteria dominated the control cultures while Clostridia comprised less than 1% of the biomass. LCFAs caused an increased in the abundance of Clostridia and Bacillus to differing degrees (PA